Merge #9294: Use internal HD chain for change outputs (hd split)

4115af7 Fix rebase issue where pwalletMain was used instead of pwallet Ser./Deser. nInternalChainCounter as last element (Jonas Schnelli)
9382f04 Do not break backward compatibility during wallet encryption (Jonas Schnelli)
1df08d1 Add assertion for CanSupportFeature(FEATURE_HD_SPLIT) (Jonas Schnelli)
cd468d0 Define CWallet::DeriveNewChildKey() as private (Jonas Schnelli)
ed79e4f Optimize GetOldestKeyPoolTime(), return as soon as we have both oldest keys (Jonas Schnelli)
771a304 Make sure we set the wallets min version to FEATURE_HD_SPLIT at the very first point (Jonas Schnelli)
1b3b5c6 Slightly modify fundrawtransaction.py test (change getnewaddress() into getrawchangeaddress()) (Jonas Schnelli)
003e197 Remove FEATURE_HD_SPLIT bump TODO (Jonas Schnelli)
d9638e5 Overhaul the internal/external key derive switch (Jonas Schnelli)
1090502 Fix superfluous cast and code style nits in RPC wallet-hd.py test (Jonas Schnelli)
58e1483 CKeyPool avoid "catch (...)" in SerializationOp (Jonas Schnelli)
e138876 Only show keypoolsize_hd_internal if HD split is enabled (Jonas Schnelli)
add38d9 GetOldestKeyPoolTime: if HD & HD Chain Split is enabled, response max(oldest-internal-key, oldest-external-key) (Jonas Schnelli)
dd526c2 Don't switch to HD-chain-split during wallet encryption of non HD-chain-split wallets (Jonas Schnelli)
79df9df Switch to 100% for the HD internal keypool size (Jonas Schnelli)
bcafca1 Make sure we always generate one keypool key at minimum (Jonas Schnelli)
d0a627a Fix issue where CDataStream->nVersion was taken a CKeyPool record version (Jonas Schnelli)
9af8f00 Make sure we hand out keypool keys if HD_SPLIT is not enabled (Jonas Schnelli)
469a47b Make sure ReserveKeyFromKeyPool only hands out internal keys if HD_SPLIT is supported (Jonas Schnelli)
05a9b49 Fix wrong keypool internal size in RPC getwalletinfo help (Jonas Schnelli)
01de822 Removed redundant IsLocked() check in NewKeyPool() (Jonas Schnelli)
d59531d Immediately return setKeyPool's size if HD or HD_SPLIT is disabled or not supported (Jonas Schnelli)
02592f4 [Wallet] split the keypool in an internal and external part (Jonas Schnelli)

Tree-SHA512: 80d355d5e844b48c3163b56c788ab8b5b5285db0ceeb19858a3ef517d5a702afeca21dbae526d7b8fb4101c2a745af1d92bf557c40cf516780f17992bf678c1a

4 files changed, 186 insertions, 74 deletions

diff --git a/src/wallet/rpcwallet.cpp b/src/wallet/rpcwallet.cpp
index 84e7eb60d7..22b1e3522c 100644
--- a/src/wallet/rpcwallet.cpp
+++ b/src/wallet/rpcwallet.cpp
@@ -221,7 +221,7 @@ UniValue getrawchangeaddress(const JSONRPCRequest& request)
 
     CReserveKey reservekey(pwallet);
     CPubKey vchPubKey;
-    if (!reservekey.GetReservedKey(vchPubKey))
+    if (!reservekey.GetReservedKey(vchPubKey, true))
         throw JSONRPCError(RPC_WALLET_KEYPOOL_RAN_OUT, "Error: Keypool ran out, please call keypoolrefill first");
 
     reservekey.KeepKey();
@@ -1852,7 +1852,7 @@ UniValue gettransaction(const JSONRPCRequest& request)
             "      \"fee\": x.xxx,                     (numeric) The amount of the fee in " + CURRENCY_UNIT + ". This is negative and only available for the \n"
             "                                           'send' category of transactions.\n"
             "      \"abandoned\": xxx                  (bool) 'true' if the transaction has been abandoned (inputs are respendable). Only available for the \n"
-            "                                           'send' category of transactions.\n"			
+            "                                           'send' category of transactions.\n"
             "    }\n"
             "    ,...\n"
             "  ],\n"
@@ -2418,16 +2418,17 @@ UniValue getwalletinfo(const JSONRPCRequest& request)
             "Returns an object containing various wallet state info.\n"
             "\nResult:\n"
             "{\n"
-            "  \"walletversion\": xxxxx,       (numeric) the wallet version\n"
-            "  \"balance\": xxxxxxx,           (numeric) the total confirmed balance of the wallet in " + CURRENCY_UNIT + "\n"
-            "  \"unconfirmed_balance\": xxx,   (numeric) the total unconfirmed balance of the wallet in " + CURRENCY_UNIT + "\n"
-            "  \"immature_balance\": xxxxxx,   (numeric) the total immature balance of the wallet in " + CURRENCY_UNIT + "\n"
-            "  \"txcount\": xxxxxxx,           (numeric) the total number of transactions in the wallet\n"
-            "  \"keypoololdest\": xxxxxx,      (numeric) the timestamp (seconds since Unix epoch) of the oldest pre-generated key in the key pool\n"
-            "  \"keypoolsize\": xxxx,          (numeric) how many new keys are pre-generated\n"
-            "  \"unlocked_until\": ttt,        (numeric) the timestamp in seconds since epoch (midnight Jan 1 1970 GMT) that the wallet is unlocked for transfers, or 0 if the wallet is locked\n"
-            "  \"paytxfee\": x.xxxx,           (numeric) the transaction fee configuration, set in " + CURRENCY_UNIT + "/kB\n"
-            "  \"hdmasterkeyid\": \"<hash160>\" (string) the Hash160 of the HD master pubkey\n"
+            "  \"walletversion\": xxxxx,          (numeric) the wallet version\n"
+            "  \"balance\": xxxxxxx,              (numeric) the total confirmed balance of the wallet in " + CURRENCY_UNIT + "\n"
+            "  \"unconfirmed_balance\": xxx,      (numeric) the total unconfirmed balance of the wallet in " + CURRENCY_UNIT + "\n"
+            "  \"immature_balance\": xxxxxx,      (numeric) the total immature balance of the wallet in " + CURRENCY_UNIT + "\n"
+            "  \"txcount\": xxxxxxx,              (numeric) the total number of transactions in the wallet\n"
+            "  \"keypoololdest\": xxxxxx,         (numeric) the timestamp (seconds since Unix epoch) of the oldest pre-generated key in the key pool\n"
+            "  \"keypoolsize\": xxxx,             (numeric) how many new keys are pre-generated (only counts external keys)\n"
+            "  \"keypoolsize_hd_internal\": xxxx, (numeric) how many new keys are pre-generated for internal use (used for change outputs, only appears if the wallet is using this feature, otherwise external keys are used)\n"
+            "  \"unlocked_until\": ttt,           (numeric) the timestamp in seconds since epoch (midnight Jan 1 1970 GMT) that the wallet is unlocked for transfers, or 0 if the wallet is locked\n"
+            "  \"paytxfee\": x.xxxx,              (numeric) the transaction fee configuration, set in " + CURRENCY_UNIT + "/kB\n"
+            "  \"hdmasterkeyid\": \"<hash160>\"     (string) the Hash160 of the HD master pubkey\n"
             "}\n"
             "\nExamples:\n"
             + HelpExampleCli("getwalletinfo", "")
@@ -2437,18 +2438,23 @@ UniValue getwalletinfo(const JSONRPCRequest& request)
     LOCK2(cs_main, pwallet->cs_wallet);
 
     UniValue obj(UniValue::VOBJ);
+
+    size_t kpExternalSize = pwallet->KeypoolCountExternalKeys();
     obj.push_back(Pair("walletversion", pwallet->GetVersion()));
     obj.push_back(Pair("balance",       ValueFromAmount(pwallet->GetBalance())));
     obj.push_back(Pair("unconfirmed_balance", ValueFromAmount(pwallet->GetUnconfirmedBalance())));
     obj.push_back(Pair("immature_balance",    ValueFromAmount(pwallet->GetImmatureBalance())));
     obj.push_back(Pair("txcount",       (int)pwallet->mapWallet.size()));
     obj.push_back(Pair("keypoololdest", pwallet->GetOldestKeyPoolTime()));
-    obj.push_back(Pair("keypoolsize",   (int)pwallet->GetKeyPoolSize()));
+    obj.push_back(Pair("keypoolsize", (int64_t)kpExternalSize));
+    CKeyID masterKeyID = pwallet->GetHDChain().masterKeyID;
+    if (!masterKeyID.IsNull() && pwallet->CanSupportFeature(FEATURE_HD_SPLIT)) {
+        obj.push_back(Pair("keypoolsize_hd_internal",   (int64_t)(pwallet->GetKeyPoolSize() - kpExternalSize)));
+    }
     if (pwallet->IsCrypted()) {
         obj.push_back(Pair("unlocked_until", pwallet->nRelockTime));
     }
     obj.push_back(Pair("paytxfee",      ValueFromAmount(payTxFee.GetFeePerK())));
-    CKeyID masterKeyID = pwallet->GetHDChain().masterKeyID;
     if (!masterKeyID.IsNull())
          obj.push_back(Pair("hdmasterkeyid", masterKeyID.GetHex()));
     return obj;
diff --git a/src/wallet/wallet.cpp b/src/wallet/wallet.cpp
index 39e2ab7c1f..e631c86bb2 100644
--- a/src/wallet/wallet.cpp
+++ b/src/wallet/wallet.cpp
@@ -85,7 +85,7 @@ const CWalletTx* CWallet::GetWalletTx(const uint256& hash) const
     return &(it->second);
 }
 
-CPubKey CWallet::GenerateNewKey()
+CPubKey CWallet::GenerateNewKey(bool internal)
 {
     AssertLockHeld(cs_wallet); // mapKeyMetadata
     bool fCompressed = CanSupportFeature(FEATURE_COMPRPUBKEY); // default to compressed public keys if we want 0.6.0 wallets
@@ -98,7 +98,7 @@ CPubKey CWallet::GenerateNewKey()
 
     // use HD key derivation if HD was enabled during wallet creation
     if (IsHDEnabled()) {
-        DeriveNewChildKey(metadata, secret);
+        DeriveNewChildKey(metadata, secret, (CanSupportFeature(FEATURE_HD_SPLIT) ? internal : false));
     } else {
         secret.MakeNewKey(fCompressed);
     }
@@ -118,13 +118,13 @@ CPubKey CWallet::GenerateNewKey()
     return pubkey;
 }
 
-void CWallet::DeriveNewChildKey(CKeyMetadata& metadata, CKey& secret)
+void CWallet::DeriveNewChildKey(CKeyMetadata& metadata, CKey& secret, bool internal)
 {
     // for now we use a fixed keypath scheme of m/0'/0'/k
     CKey key;                      //master key seed (256bit)
     CExtKey masterKey;             //hd master key
     CExtKey accountKey;            //key at m/0'
-    CExtKey externalChainChildKey; //key at m/0'/0'
+    CExtKey chainChildKey;         //key at m/0'/0' (external) or m/0'/1' (internal)
     CExtKey childKey;              //key at m/0'/0'/<n>'
 
     // try to get the master key
@@ -137,22 +137,28 @@ void CWallet::DeriveNewChildKey(CKeyMetadata& metadata, CKey& secret)
     // use hardened derivation (child keys >= 0x80000000 are hardened after bip32)
     masterKey.Derive(accountKey, BIP32_HARDENED_KEY_LIMIT);
 
-    // derive m/0'/0'
-    accountKey.Derive(externalChainChildKey, BIP32_HARDENED_KEY_LIMIT);
+    // derive m/0'/0' (external chain) OR m/0'/1' (internal chain)
+    assert(internal ? CanSupportFeature(FEATURE_HD_SPLIT) : true);
+    accountKey.Derive(chainChildKey, BIP32_HARDENED_KEY_LIMIT+(internal ? 1 : 0));
 
     // derive child key at next index, skip keys already known to the wallet
     do {
         // always derive hardened keys
         // childIndex | BIP32_HARDENED_KEY_LIMIT = derive childIndex in hardened child-index-range
         // example: 1 | BIP32_HARDENED_KEY_LIMIT == 0x80000001 == 2147483649
-        externalChainChildKey.Derive(childKey, hdChain.nExternalChainCounter | BIP32_HARDENED_KEY_LIMIT);
-        metadata.hdKeypath = "m/0'/0'/" + std::to_string(hdChain.nExternalChainCounter) + "'";
-        metadata.hdMasterKeyID = hdChain.masterKeyID;
-        // increment childkey index
-        hdChain.nExternalChainCounter++;
+        if (internal) {
+            chainChildKey.Derive(childKey, hdChain.nInternalChainCounter | BIP32_HARDENED_KEY_LIMIT);
+            metadata.hdKeypath = "m/0'/1'/" + std::to_string(hdChain.nInternalChainCounter) + "'";
+            hdChain.nInternalChainCounter++;
+        }
+        else {
+            chainChildKey.Derive(childKey, hdChain.nExternalChainCounter | BIP32_HARDENED_KEY_LIMIT);
+            metadata.hdKeypath = "m/0'/0'/" + std::to_string(hdChain.nExternalChainCounter) + "'";
+            hdChain.nExternalChainCounter++;
+        }
     } while (HaveKey(childKey.key.GetPubKey().GetID()));
     secret = childKey.key;
-
+    metadata.hdMasterKeyID = hdChain.masterKeyID;
     // update the chain model in the database
     if (!CWalletDB(strWalletFile).WriteHDChain(hdChain))
         throw std::runtime_error(std::string(__func__) + ": Writing HD chain model failed");
@@ -633,7 +639,9 @@ bool CWallet::EncryptWallet(const SecureString& strWalletPassphrase)
         if (IsHDEnabled()) {
             CKey key;
             CPubKey masterPubKey = GenerateNewHDMasterKey();
-            if (!SetHDMasterKey(masterPubKey))
+            // preserve the old chains version to not break backward compatibility
+            CHDChain oldChain = GetHDChain();
+            if (!SetHDMasterKey(masterPubKey, &oldChain))
                 return false;
         }
 
@@ -799,7 +807,7 @@ bool CWallet::GetAccountPubkey(CPubKey &pubKey, std::string strAccount, bool bFo
 
     // Generate a new key
     if (bForceNew) {
-        if (!GetKeyFromPool(account.vchPubKey))
+        if (!GetKeyFromPool(account.vchPubKey, false))
             return false;
 
         SetAddressBook(account.vchPubKey.GetID(), strAccount, "receive");
@@ -1300,17 +1308,17 @@ CPubKey CWallet::GenerateNewHDMasterKey()
     return pubkey;
 }
 
-bool CWallet::SetHDMasterKey(const CPubKey& pubkey)
+bool CWallet::SetHDMasterKey(const CPubKey& pubkey, CHDChain *possibleOldChain)
 {
     LOCK(cs_wallet);
-
-    // ensure this wallet.dat can only be opened by clients supporting HD
-    SetMinVersion(FEATURE_HD);
-
     // store the keyid (hash160) together with
     // the child index counter in the database
     // as a hdchain object
     CHDChain newHdChain;
+    if (possibleOldChain) {
+        // preserve the old chains version
+        newHdChain.nVersion = possibleOldChain->nVersion;
+    }
     newHdChain.masterKeyID = pubkey.GetID();
     SetHDChain(newHdChain, false);
 
@@ -2445,7 +2453,7 @@ bool CWallet::CreateTransaction(const std::vector<CRecipient>& vecSend, CWalletT
                         // Reserve a new key pair from key pool
                         CPubKey vchPubKey;
                         bool ret;
-                        ret = reservekey.GetReservedKey(vchPubKey);
+                        ret = reservekey.GetReservedKey(vchPubKey, true);
                         if (!ret)
                         {
                             strFailReason = _("Keypool ran out, please call keypoolrefill first");
@@ -2896,21 +2904,37 @@ bool CWallet::NewKeyPool()
             walletdb.ErasePool(nIndex);
         setKeyPool.clear();
 
-        if (IsLocked())
+        if (!TopUpKeyPool()) {
             return false;
-
-        int64_t nKeys = std::max(GetArg("-keypool", DEFAULT_KEYPOOL_SIZE), (int64_t)0);
-        for (int i = 0; i < nKeys; i++)
-        {
-            int64_t nIndex = i+1;
-            walletdb.WritePool(nIndex, CKeyPool(GenerateNewKey()));
-            setKeyPool.insert(nIndex);
         }
-        LogPrintf("CWallet::NewKeyPool wrote %d new keys\n", nKeys);
+        LogPrintf("CWallet::NewKeyPool rewrote keypool\n");
     }
     return true;
 }
 
+size_t CWallet::KeypoolCountExternalKeys()
+{
+    AssertLockHeld(cs_wallet); // setKeyPool
+
+    // immediately return setKeyPool's size if HD or HD_SPLIT is disabled or not supported
+    if (!IsHDEnabled() || !CanSupportFeature(FEATURE_HD_SPLIT))
+        return setKeyPool.size();
+
+    CWalletDB walletdb(strWalletFile);
+
+    // count amount of external keys
+    size_t amountE = 0;
+    for(const int64_t& id : setKeyPool)
+    {
+        CKeyPool tmpKeypool;
+        if (!walletdb.ReadPool(id, tmpKeypool))
+            throw std::runtime_error(std::string(__func__) + ": read failed");
+        amountE += !tmpKeypool.fInternal;
+    }
+
+    return amountE;
+}
+
 bool CWallet::TopUpKeyPool(unsigned int kpSize)
 {
     {
@@ -2919,8 +2943,6 @@ bool CWallet::TopUpKeyPool(unsigned int kpSize)
         if (IsLocked())
             return false;
 
-        CWalletDB walletdb(strWalletFile);
-
         // Top up key pool
         unsigned int nTargetSize;
         if (kpSize > 0)
@@ -2928,21 +2950,37 @@ bool CWallet::TopUpKeyPool(unsigned int kpSize)
         else
             nTargetSize = std::max(GetArg("-keypool", DEFAULT_KEYPOOL_SIZE), (int64_t) 0);
 
-        while (setKeyPool.size() < (nTargetSize + 1))
+        // count amount of available keys (internal, external)
+        // make sure the keypool of external and internal keys fits the user selected target (-keypool)
+        int64_t amountExternal = KeypoolCountExternalKeys();
+        int64_t amountInternal = setKeyPool.size() - amountExternal;
+        int64_t missingExternal = std::max(std::max((int64_t) nTargetSize, (int64_t) 1) - amountExternal, (int64_t) 0);
+        int64_t missingInternal = std::max(std::max((int64_t) nTargetSize, (int64_t) 1) - amountInternal, (int64_t) 0);
+
+        if (!IsHDEnabled() || !CanSupportFeature(FEATURE_HD_SPLIT))
+        {
+            // don't create extra internal keys
+            missingInternal = 0;
+        }
+        bool internal = false;
+        CWalletDB walletdb(strWalletFile);
+        for (int64_t i = missingInternal + missingExternal; i--;)
         {
             int64_t nEnd = 1;
+            if (i < missingInternal)
+                internal = true;
             if (!setKeyPool.empty())
                 nEnd = *(--setKeyPool.end()) + 1;
-            if (!walletdb.WritePool(nEnd, CKeyPool(GenerateNewKey())))
+            if (!walletdb.WritePool(nEnd, CKeyPool(GenerateNewKey(internal), internal)))
                 throw std::runtime_error(std::string(__func__) + ": writing generated key failed");
             setKeyPool.insert(nEnd);
-            LogPrintf("keypool added key %d, size=%u\n", nEnd, setKeyPool.size());
+            LogPrintf("keypool added key %d, size=%u, internal=%d\n", nEnd, setKeyPool.size(), internal);
         }
     }
     return true;
 }
 
-void CWallet::ReserveKeyFromKeyPool(int64_t& nIndex, CKeyPool& keypool)
+void CWallet::ReserveKeyFromKeyPool(int64_t& nIndex, CKeyPool& keypool, bool internal)
 {
     nIndex = -1;
     keypool.vchPubKey = CPubKey();
@@ -2958,14 +2996,24 @@ void CWallet::ReserveKeyFromKeyPool(int64_t& nIndex, CKeyPool& keypool)
 
         CWalletDB walletdb(strWalletFile);
 
-        nIndex = *(setKeyPool.begin());
-        setKeyPool.erase(setKeyPool.begin());
-        if (!walletdb.ReadPool(nIndex, keypool))
-            throw std::runtime_error(std::string(__func__) + ": read failed");
-        if (!HaveKey(keypool.vchPubKey.GetID()))
-            throw std::runtime_error(std::string(__func__) + ": unknown key in key pool");
-        assert(keypool.vchPubKey.IsValid());
-        LogPrintf("keypool reserve %d\n", nIndex);
+        // try to find a key that matches the internal/external filter
+        for(const int64_t& id : setKeyPool)
+        {
+            CKeyPool tmpKeypool;
+            if (!walletdb.ReadPool(id, tmpKeypool))
+                throw std::runtime_error(std::string(__func__) + ": read failed");
+            if (!HaveKey(tmpKeypool.vchPubKey.GetID()))
+                throw std::runtime_error(std::string(__func__) + ": unknown key in key pool");
+            if (!IsHDEnabled() || !CanSupportFeature(FEATURE_HD_SPLIT) || tmpKeypool.fInternal == internal)
+            {
+                nIndex = id;
+                keypool = tmpKeypool;
+                setKeyPool.erase(id);
+                assert(keypool.vchPubKey.IsValid());
+                LogPrintf("keypool reserve %d\n", nIndex);
+                return;
+            }
+        }
     }
 }
 
@@ -2990,17 +3038,17 @@ void CWallet::ReturnKey(int64_t nIndex)
     LogPrintf("keypool return %d\n", nIndex);
 }
 
-bool CWallet::GetKeyFromPool(CPubKey& result)
+bool CWallet::GetKeyFromPool(CPubKey& result, bool internal)
 {
     int64_t nIndex = 0;
     CKeyPool keypool;
     {
         LOCK(cs_wallet);
-        ReserveKeyFromKeyPool(nIndex, keypool);
+        ReserveKeyFromKeyPool(nIndex, keypool, internal);
         if (nIndex == -1)
         {
             if (IsLocked()) return false;
-            result = GenerateNewKey();
+            result = GenerateNewKey(internal);
             return true;
         }
         KeepKey(nIndex);
@@ -3017,9 +3065,33 @@ int64_t CWallet::GetOldestKeyPoolTime()
     if (setKeyPool.empty())
         return GetTime();
 
-    // load oldest key from keypool, get time and return
     CKeyPool keypool;
     CWalletDB walletdb(strWalletFile);
+
+    if (IsHDEnabled() && CanSupportFeature(FEATURE_HD_SPLIT))
+    {
+        // if HD & HD Chain Split is enabled, response max(oldest-internal-key, oldest-external-key)
+        int64_t now = GetTime();
+        int64_t oldest_external = now, oldest_internal = now;
+
+        for(const int64_t& id : setKeyPool)
+        {
+            if (!walletdb.ReadPool(id, keypool)) {
+                throw std::runtime_error(std::string(__func__) + ": read failed");
+            }
+            if (keypool.fInternal && keypool.nTime < oldest_internal) {
+                oldest_internal = keypool.nTime;
+            }
+            else if (!keypool.fInternal && keypool.nTime < oldest_external) {
+                oldest_external = keypool.nTime;
+            }
+            if (oldest_internal != now && oldest_external != now) {
+                break;
+            }
+        }
+        return std::max(oldest_internal, oldest_external);
+    }
+    // load oldest key from keypool, get time and return
     int64_t nIndex = *(setKeyPool.begin());
     if (!walletdb.ReadPool(nIndex, keypool))
         throw std::runtime_error(std::string(__func__) + ": read oldest key in keypool failed");
@@ -3205,12 +3277,12 @@ std::set<CTxDestination> CWallet::GetAccountAddresses(const std::string& strAcco
     return result;
 }
 
-bool CReserveKey::GetReservedKey(CPubKey& pubkey)
+bool CReserveKey::GetReservedKey(CPubKey& pubkey, bool internal)
 {
     if (nIndex == -1)
     {
         CKeyPool keypool;
-        pwallet->ReserveKeyFromKeyPool(nIndex, keypool);
+        pwallet->ReserveKeyFromKeyPool(nIndex, keypool, internal);
         if (nIndex != -1)
             vchPubKey = keypool.vchPubKey;
         else {
@@ -3623,13 +3695,17 @@ CWallet* CWallet::CreateWalletFromFile(const std::string walletFile)
     {
         // Create new keyUser and set as default key
         if (GetBoolArg("-usehd", DEFAULT_USE_HD_WALLET) && !walletInstance->IsHDEnabled()) {
+
+            // ensure this wallet.dat can only be opened by clients supporting HD with chain split
+            walletInstance->SetMinVersion(FEATURE_HD_SPLIT);
+
             // generate a new master key
             CPubKey masterPubKey = walletInstance->GenerateNewHDMasterKey();
             if (!walletInstance->SetHDMasterKey(masterPubKey))
                 throw std::runtime_error(std::string(__func__) + ": Storing master key failed");
         }
         CPubKey newDefaultKey;
-        if (walletInstance->GetKeyFromPool(newDefaultKey)) {
+        if (walletInstance->GetKeyFromPool(newDefaultKey, false)) {
             walletInstance->SetDefaultKey(newDefaultKey);
             if (!walletInstance->SetAddressBook(walletInstance->vchDefaultKey.GetID(), "", "receive")) {
                 InitError(_("Cannot write default address") += "\n");
@@ -3888,12 +3964,14 @@ bool CWallet::BackupWallet(const std::string& strDest)
 CKeyPool::CKeyPool()
 {
     nTime = GetTime();
+    fInternal = false;
 }
 
-CKeyPool::CKeyPool(const CPubKey& vchPubKeyIn)
+CKeyPool::CKeyPool(const CPubKey& vchPubKeyIn, bool internalIn)
 {
     nTime = GetTime();
     vchPubKey = vchPubKeyIn;
+    fInternal = internalIn;
 }
 
 CWalletKey::CWalletKey(int64_t nExpires)
diff --git a/src/wallet/wallet.h b/src/wallet/wallet.h
index ae4321eef8..ccede60097 100644
--- a/src/wallet/wallet.h
+++ b/src/wallet/wallet.h
@@ -86,6 +86,9 @@ enum WalletFeature
     FEATURE_COMPRPUBKEY = 60000, // compressed public keys
 
     FEATURE_HD = 130000, // Hierarchical key derivation after BIP32 (HD Wallet)
+
+    FEATURE_HD_SPLIT = 139900, // Wallet with HD chain split (change outputs will use m/0'/1'/k)
+
     FEATURE_LATEST = FEATURE_COMPRPUBKEY // HD is optional, use FEATURE_COMPRPUBKEY as latest version
 };
 
@@ -96,9 +99,10 @@ class CKeyPool
 public:
     int64_t nTime;
     CPubKey vchPubKey;
+    bool fInternal; // for change outputs
 
     CKeyPool();
-    CKeyPool(const CPubKey& vchPubKeyIn);
+    CKeyPool(const CPubKey& vchPubKeyIn, bool internalIn);
 
     ADD_SERIALIZE_METHODS;
 
@@ -109,6 +113,19 @@ public:
             READWRITE(nVersion);
         READWRITE(nTime);
         READWRITE(vchPubKey);
+        if (ser_action.ForRead()) {
+            try {
+                READWRITE(fInternal);
+            }
+            catch (std::ios_base::failure&) {
+                /* flag as external address if we can't read the internal boolean
+                   (this will be the case for any wallet before the HD chain split version) */
+                fInternal = false;
+            }
+        }
+        else {
+            READWRITE(fInternal);
+        }
     }
 };
 
@@ -647,6 +664,9 @@ private:
     /* the HD chain data model (external chain counters) */
     CHDChain hdChain;
 
+    /* HD derive new child key (on internal or external chain) */
+    void DeriveNewChildKey(CKeyMetadata& metadata, CKey& secret, bool internal = false);
+
     bool fFileBacked;
 
     std::set<int64_t> setKeyPool;
@@ -774,8 +794,7 @@ public:
      * keystore implementation
      * Generate a new key
      */
-    CPubKey GenerateNewKey();
-    void DeriveNewChildKey(CKeyMetadata& metadata, CKey& secret);
+    CPubKey GenerateNewKey(bool internal = false);
     //! Adds a key to the store, and saves it to disk.
     bool AddKeyPubKey(const CKey& key, const CPubKey &pubkey) override;
     //! Adds a key to the store, without saving it to disk (used by LoadWallet)
@@ -883,11 +902,12 @@ public:
     static CAmount GetRequiredFee(unsigned int nTxBytes);
 
     bool NewKeyPool();
+    size_t KeypoolCountExternalKeys();
     bool TopUpKeyPool(unsigned int kpSize = 0);
-    void ReserveKeyFromKeyPool(int64_t& nIndex, CKeyPool& keypool);
+    void ReserveKeyFromKeyPool(int64_t& nIndex, CKeyPool& keypool, bool internal);
     void KeepKey(int64_t nIndex);
     void ReturnKey(int64_t nIndex);
-    bool GetKeyFromPool(CPubKey &key);
+    bool GetKeyFromPool(CPubKey &key, bool internal = false);
     int64_t GetOldestKeyPoolTime();
     void GetAllReserveKeys(std::set<CKeyID>& setAddress) const;
 
@@ -1035,8 +1055,10 @@ public:
     /* Generates a new HD master key (will not be activated) */
     CPubKey GenerateNewHDMasterKey();
     
-    /* Set the current HD master key (will reset the chain child index counters) */
-    bool SetHDMasterKey(const CPubKey& key);
+    /* Set the current HD master key (will reset the chain child index counters)
+       If possibleOldChain is provided, the parameters from the old chain (version)
+       will be preserved. */
+    bool SetHDMasterKey(const CPubKey& key, CHDChain *possibleOldChain = nullptr);
 };
 
 /** A key allocated from the key pool. */
@@ -1063,7 +1085,7 @@ public:
     }
 
     void ReturnKey();
-    bool GetReservedKey(CPubKey &pubkey);
+    bool GetReservedKey(CPubKey &pubkey, bool internal = false);
     void KeepKey();
     void KeepScript() { KeepKey(); }
 };
diff --git a/src/wallet/walletdb.h b/src/wallet/walletdb.h
index 4d7dfb727e..271c1d66c9 100644
--- a/src/wallet/walletdb.h
+++ b/src/wallet/walletdb.h
@@ -46,9 +46,12 @@ class CHDChain
 {
 public:
     uint32_t nExternalChainCounter;
+    uint32_t nInternalChainCounter;
     CKeyID masterKeyID; //!< master key hash160
 
-    static const int CURRENT_VERSION = 1;
+    static const int VERSION_HD_BASE        = 1;
+    static const int VERSION_HD_CHAIN_SPLIT = 2;
+    static const int CURRENT_VERSION        = VERSION_HD_CHAIN_SPLIT;
     int nVersion;
 
     CHDChain() { SetNull(); }
@@ -59,12 +62,15 @@ public:
         READWRITE(this->nVersion);
         READWRITE(nExternalChainCounter);
         READWRITE(masterKeyID);
+        if (this->nVersion >= VERSION_HD_CHAIN_SPLIT)
+            READWRITE(nInternalChainCounter);
     }
 
     void SetNull()
     {
         nVersion = CHDChain::CURRENT_VERSION;
         nExternalChainCounter = 0;
+        nInternalChainCounter = 0;
         masterKeyID.SetNull();
     }
 };